1. Field of the Invention
This invention relates to a wire bonding process for use in forming a substrate based package and, more particularly, to a wire bonding process equipped with a function of automatically skipping defective work pieces.
2. Description of the Related Art
Conventional substrate based packages such as the plastic BGA package 100 (see FIG. 1) typically comprises a substrate 110 having a die covering area 112 (referring to FIG. 2) on the upper surface thereof and a die 120 disposed on the die covering area 112. The upper surface of the substrate 110 is provided with a paddle 114 and a plurality of conductive traces 116 around the paddle 114 (see FIG. 2). Referring to FIG. 2, the die covering area 112 is defined inside the paddle 114 and the distance from the boundary of the die covering area 112 to the boundary of the paddle 114 is about 15 mils. It is noted that when die 120 is mounted to the paddle 114 of the substrate 110, the die covering area 112 will be entirely covered by the die 120.
Referring to FIG. 1 and FIG. 2, the active surface of the die 120 is provided with a plurality of bonding pads connected to the conductive traces 116 of the substrate 110 through a plurality of bonding wires 124. The lower surface of the substrate 110 is provided with a plurality of solder pads 118 electrically connected to corresponding conductive traces 116. Each solder pad 118 is provided with a solder ball 130 for making external electrical connection. The paddle 114, the conductive traces 116 and the solder pads 118 are usually made of metal with good electrical conductivity such as copper.
Typically, the substrate 110 is provided with a layer of solder mask 110a covering the surface thereof wherein lead fingers 117 of the conductive traces 116 adapted for connecting to the bonding wires 124 and the solder pads 118 are exposed from the solder mask 110a. In addition, the paddle 114 typically has some portions exposed from the solder mask 110a so as to form a plurality of thermal pads 113 used for heat transfer. Further, the upper surface of the substrate 110 is usually provided with several lead eyes 119 disposed outside the die covering area 112 wherein the lead eyes 119 are also exposed from the solder mask 110a. All of the surfaces of the thermal pads 113, the lead fingers 117, the solder pads 118, and the lead eyes 119 are plated with a layer of gold or palladium for preventing oxidation and increasing bondability.
In production, it is desirable to integrally form a plurality of substrates in a strip (typically referred to as a xe2x80x9csubstrate stripxe2x80x9d) having alignment holes so that the packaging process can be automated. In a conventional packaging process, the die 120 is attached to the die covering area 112 on the paddle 114. Then, during a wire bonding operation, the bond wires 124 are connected to the bonding pads on the die 120 and the lead fingers 117 on the substrate 110. Finally, the dies 120, bonding wires 124, and a portion of the substrate 110 including majority of the conductive traces 116 are encapsulated in a package body 140. The present invention is directed to the wire bonding operation.
FIG. 3 shows a wire bonder with a substrate strip 200 installed therein illustrating conventional wire bonding operation. Usually, defective substrates of the substrate strip 200 are marked with white ink 210 so that defective substrates can be distinguished from other normal substrates. Therefore, only normal substrates of the substrate strip 200 have dies 120 mounted thereon. On the contrary, defective substrates are just dispensed with precise amount of epoxy but without dies 120 placed thereon. When the substrate strip 200 is indexed to properly position each die 120 under a bond head 220, a camera (not shown), integrated into the bond head 220 and connected to a recognition system (not shown), will try to find and align the reference structures such as the lead eye 119 on the substrate or the die eye formed on the active surface of the die. When a defective substrate with white ink 210 is indexed to the bond head 220, the wire bonder will emit an alarm to notify an operator because the camera cannot find the lead eye (masked by the white ink 210) and the die eye (no die placed thereon). Then the operator will check the wire bonder and trigger a procedure for skipping the defective substrate, i. e., indexing the next to-be-bonded substrate to the bond head and re-performing the wire bonding operation.
However, in the conventional wire bonding operation described above, skipping of defective work pieces has only been dealt with manually in the past, causing significant operating down time and reduction of throughput. Moreover, when (a) the solder mask 110a is imaged and developed not so precisely as to cover a portion of the lead eye on the substrate or (b) the die eye on the semiconductor die becomes faint, the contour of the lead eye or the die eye will change a little such that it is easy to evaluate the lead eye or the die eye as not being present. This will lead to a result that work pieces are not defective but still skipped from wire-bonding.
Therefore, the present invention seeks to provide a method of automatically identifying and skipping defective work pieces which overcomes, or at least reduces the above-mentioned problems of the prior art.
It is a primary object of the present invention to provide a method of automatically identifying and skipping defective work pieces from normal work pieces which utilizes a reject eye formed inside the die covering area on a substrate to automatically determine whether the skipping procedure is triggered or not thereby reducing operating down time and increasing throughput.
It is another object of the present invention to provide a substrate for use in packaging of a semiconductor die, the substrate comprising at least a reject eye formed inside the die covering area of a substrate. The reject eye is adapted to be identified by a recognition system which gives a signal of stopping a wire bonding operation and triggering a skipping procedure when the reject eye is located on condition that no semiconductor die is disposed on the die covering area of the substrate.
The method of automatically identifying and skipping defective work pieces from normal work pieces used in a wire bonding operation in accordance with the present invention comprises: (a) finding and aligning the die eye of the die as well as the lead eye of the substrate; (b1) proceeding the wire bonding operation when the die eye and the lead eye is located; (b2) finding the reject eye when the die eye and the lead eye is evaluated as not being present; (c1) stopping the wire bonding operation and skipping to next work piece when the reject eye is located; (c2) stopping the wire bonding process and notifying an operator when the reject eye is evaluated as not being present; (c2xe2x80x2) re-finding and re-aligning the die eye of the die and the lead eye of the substrate when the reject eye is evaluated as not being present. The method of skipping defective work pieces of the present invention is capable of automatically determining whether the skipping procedure is triggered or not thereby reducing operating down time and increasing throughput.
The substrate for use in packaging of a semiconductor die in accordance with the present invention comprising a metal paddle disposed in the central region of the substrate a solder mask formed over the metal paddle wherein at least a portion of the metal paddle is exposed from the solder mask to form an irregular mark. The substrate has a die covering area defined inside the metal paddle for receiving a semiconductor die and an epoxy area adapted for epoxy to dispense thereon defined inside the die covering area wherein the irregular mark is located inside the die covering area but outside the epoxy area. The irregular mark can be set as an eye point (referred to as a xe2x80x9creject eyexe2x80x9d in the present invention) capable of being located by a recognition system integrated inside a wire bonder. The recognition system will give a signal of stopping a wire bonding operation and triggering the skipping procedure when the reject eye is located on condition that no die is placed on the substrate. The reject eye in accordance with the present invention is set on the irregular mark formed from the exposed portions of the metal paddle. Therefore, even if the solder mask is imaged and developed not very precisely, the contour of the irregular mark will still remain the same thereby preventing misevaluating the reject eye as not being present and assuring wire-bonding of normal substrates.